The present invention relates to a size press. More particularly, the invention relates to a size press with a coating pool suppressing arrangement.
The size press comprises two or more coating rollers which form one or a plurality of coating nips with each other. The web to be treated is arranged to run through the coating nip or nips, preferably in a main direction from up to down. The size press further comprises coating supply apparatus for supplying a coating pool or pools at the nip entrance between the coating roller and the web entering the coating nip or nips.
The surface sizing of paper is usually made by an "on-machine" size press disposed in the paper machine. Several different operations affecting the properties of paper and board may be carried out by the size press.
As known, the paper web is run through the roller nip of the size press. The coating agent is supplied to paper in the press by forming a pool or pools in front of the nip. The web gets wet in the pool or pools and adsorbs the coating agent on itself. The coating agent is pressed into the web between the rollers of the size press, due to hydrodynamic pressure. A film of the coating agent is simultaneously formed, due to hydrodynamic pressure, between the paper and the roller, splitting at the nip exit, thereby leaving a layer of coating agent on the surface of the paper. The thickness of the coating agent layer depends on the viscosity of the coating agent, the speed of the machine, the diameter of the roller, the surface characteristics of the roller, the linear load and factors caused by the transformation of the roller.
As known, the oldest of size presses is the so-called vertical size press, in which the rollers are placed one on top of the other and the paper runs horizontally between said rollers. In the known horizontal size press, the rollers are placed in the same horizontal plane and the paper runs vertically between said rollers. A third type of known size press is the so-called oblique press, in which the rollers are placed one on top of the other at an angle of about 45.degree.. This is the best known solution regarding the running of the web at high operating speeds.
As known, the web is led into the size press by one or two guide rollers, of which the distance of that closest to the press nip from the press is important. The last-mentioned guide roller is adjustable and, via means for moving said roller, it is possible to adjust the coating pool passing time, that is, the adsorption time. It is also possible to treat only one side of the paper in the size press.
In the known size presses, the tail of the web is carried by ropes, and the tail is released from the ropes as soon as possible. A paper widening roller is used in front of the drying cylinder, in order to avoid wrinkles.
A serious problem in size presses is the splashing of the sizing or equivalent coating agent. The disadvantages of splashing are accentuated when the speeds of the size presses are increased. Splashing is especially considerable in "on-machine" size presses.
The diameters of the rollers of the size press are important regarding the suitability to operation. In the known size presses, the diameters of the nip rollers are fixed to about 800-1500 mm at operating speeds of 10-17 m/s. As known, it is attempted, by increasing the roller diameter as the speed increases, to prevent splashing in the pool entrance, because this disturbs the even wetting of the paper web.
As known, rubber is primarily used as the covering material of the rollers of the size press. Known size presses utilize a hard roller of metal and soft roller covered by rubber. Thus, softer and harder pressing zones are used. The diameters of the rollers are increasing as the operating speeds and the widths of the machines are changing. Not only is a sufficient wetting time required in order to achieve a good starch penetration, but also a high pressing load, 40-50 kN/m.
The most common surface sizing agent used for increasing the strength of the web is starch. Lesser used bonding agents are CMC and different synthetic bonding agents such as PVA. Furthermore, porosity decreasing agents, which are often simultaneously also bonding agents, are used. Several different paper additives, which may be applied to paper by the size press, are used as other surface treatment agents for paper or board.
The size press is suitable as the means for applying different additives due to the fact that the retention to paper is 100%, due to the use of the size press. It is possible to apply the coating onto the surface of the paper only, even onto one surface only, if necessary, due to the use of the size press. Plain water is sometimes also used in the size press for improving the moisture profile or for improving the dimension stability.
The production of fine paper is shifting to large paper machines, which imposes great requirements on the size presses. A poorly operating size press decreases the productivity of the entire paper machine. Thus, the size press of the invention presents some solutions that have been developed especially regarding high operating speeds.
Although the term "size press" is used herein, it must be understood that in this context the term "coating agent" is intended to have a wide meaning, so that it also comprises other known treatment agents, for paper or board webs, which are suitable for use in apparatus similar to that disclosed.
It must also be emphasized that the size press of the invention is suitable for use as a so-called "off-machine" coating machine.
As hereinbefore mentioned, a serious disadvantage of known size presses and corresponding coating machines is the splashing of the sizing agent or corresponding coating agent. Such splashing may even totally spoil the web being produced. The disadvantages caused by splashing have been accentuated as the speeds of size presses have increased. Splashing problems have thus been especially marked in on-machine size presses.
In the known size presses, splashing has occurred especially in coating pools. It has been attempted to avoid this disadvantage by arrangements, disclosed in U.S. Pat. Nos. 3,018,757 and 3,150,997, using plates with dense covering, pipes or rods. However, the devices of these patents for suppressing the coating pool, are so placed relative to the pool that there is no direct connection of the pool with the web. The coating agent is supplied by a roller connected to the pool through the surface of the roller into the nip through which the web runs. In such known coating supply apparatus, the splashing of the coating agent is not such an accentuated problem as in coating machines in which the web runs directly through the coating pool.
As known, it has been attempted to avoid the aforementioned splashing problems by using large rollers, so that a sufficiently small radial acceleration is achieved by the rollers. However, at high web speeds this arrangement leads to unreasonably large roller diameters. The large roller diameters again lead to an expensive space-consuming structure.